Electrostatic precipitators are effective devices for cleaning fine and ultrafine aerosols. Electrostatic precipitators have several advantages over gas cleaning systems of different technology: They need less energy than mechanical collection systems and have no moving parts; maintenance costs are low and downtimes are reduced.
The design of a compact, highly efficient electrostatic precipitator for droplet aerosols is described in U.S. Pat. No. 6,221,136. This electrostatic precipitator has a high-voltage electrode including multiple wire segments that are positioned within an electrically conductive porous medium and have a central axis along which the electrode assembly extends. The electrode assembly includes a plurality of wire lengths positioned to extend in a direction along the longitudinal axis of the porous medium. The wire segments are arranged to have a substantially longer total length than the length of extension along the longitudinal axis. The particles are passed through the porous medium and across the electrode, and are charged by the high voltage. The porous medium is at a substantially lower voltage than the high-voltage electrode. The flow of the aerosol charged at the electrodes passes through the porous medium to the outlet, in which process the charged particles are precipitated by the porous medium. Electrostatic shields are provided around the high-voltage insulators to reduce the likelihood of contamination of the insulators, which causes current leakage.
Despite this design, this precipitator has several problems. First, when processing sticky aerosols, the electrodes become covered with particles, resulting in a reduction in the efficiency of the precipitator. Second, the insulator is positioned within the collector, where the charged particles are present and form the space charge. A portion of the charged droplets may deposit on the insulator surface under the influence of the space charge, resulting in contamination of the insulator surface. Third, the distance between the electrostatic shields and the housing of the precipitator is small. Therefore, flashovers may occur within the precipitator when the shields become covered with particles. The spark discharges reduce the efficiency of the collector. The porous medium forming the collector performs two functions: First, it is used as a grounded electrode. Second, it collects aerosol particles, which may be in the form of droplets or solid particles. If the filter surface becomes covered with a dielectric fluid, such as lubricating oil, the electric field strength in the electrode system will decrease, reducing the particle charging efficiency.
These problems are substantially eliminated by the measures described in DE 102 44 051 and DE 10 2004 037 286. Document DE 102 44 051 describes an electrostatic precipitator including an ionizer having a plurality of needle- or star-shaped electrodes installed downstream in a grounded nozzle plate. The charged particles are collected in a collector installed downstream of the ionizer (DE 102 44 051 and DE 10 2004 037 286). Due to the small distance between the high-voltage electrode and the grounded electrode in the electrode system, a strong electric field is present in the region of charged particles. Compared to conventional electrostatic precipitators, this makes it possible to operate at high voltages of relatively low magnitude (<20 kV) for charging the particles. The gas stream flows at high velocity through the ionizer and at low velocity through the collector, which is the actual filter. The high velocity of the gas stream in the ionizer stabilizes the operation of the electrostatic precipitator, decreases the influence of the space charge on the charged particles, and reduces corona discharge suppression. The low velocity in the collector improves its efficiency and reduces the pressure drop therein. The grounded electrode in the electrode system and the collector are spatially separated from one another. This reduces clogging of the collector. The grounded grid/mesh electrode or nozzle lets the charged aerosol particles pass therethrough. The electric wind can pass through the mesh electrode without pressure drop. The use of star-shaped electrodes and the high velocity in the electrode region reduces the deposition of sticky particles or droplets on the high-voltage electrodes.
Despite these improvements in the efficiency of the charging and precipitation of particles, the use of an operating high voltage of low magnitude, the operational stability achieved by corona suppression and the avoidance of deposits on the electrode system, the precipitator is relatively voluminous because of the spatial separation of the ionization stage from the collector. The high-voltage insulator is positioned in the raw gas or in the clean gas stream, wherefore additional measures must be taken against contamination.